Method of treating aluminium-silicon alloys



Patented May 4, 1926.

' UNITED STATES PATENT OFFICE.

AILFBED GEORGE COOPER GWYER AND HENRY WILFBED LEWIS PHILLII'S, OF IiON-DON, ENGLAND, ASSIGNORS TO THE BRITISH, ALUMINIUM COMPANY, LIMITED,

01* LONDON, ENGLAND.

METHOD or TREATING ALUMINUM-SILICON ALLOYS.

I No Drawing.

To all whom it may concern:

Be it known that we, 'ALFRED GEORGE COOPER GWYER, a subject of the Kingof Great Britain and Ireland, and HENRY VVILFRED LEwIs PHILLIPS, asubject of the King of Great Britain and Ireland, and

both residing at 109 Queen Victoria Street,

In alloys containing aluminum and silicon the silicon, if present to theextent of about 11 to 15 per cent, may exist in'the form of more ,orless large crystals, or it may be widely dispersed in the eutectic,while inalloys wit-h less than about 11 per cent Sill? con the siliconis in either case all in the eutectic, though again its dispersion mayvary between very wide limits, the degree of dispersion being, ofcourse, greatest in the completely modified and least in the completelynormal alloys. The greater dispersion is accompanied by improvedphysical .properties in the alloy.

We have made a large number of experi-. ments in connection with theimprovement of the structure of alloys and have found as the result ofour investigations that the mechanism of the process known asmodivfication issuch that the phenomena concerned are not solely' andexclusively characteristic of a particular series of alloys but theprocess of modification is capable of being applied and extended to alarge number of or many alloy systems.

We have formulated a theory which we now put forward to assist in theunderstanding of our invention as we find it useful to that end, whetherindeed it includes the true explanation of what happens or not.

According to this theory we assume that certain alloys when'in themolteri'state and 4 above a certain temperature consist of homogeneous,liquids of which the constituent .or colloid protector Application filedJune 9, 1925. Serial to. 36,043.

metals are in a state of molecular dispersion (1. e., true solution),but that below a certain temperature which is probably not much abovethe freezing point the molecules become associated in groups ofcolloidal particles. With ordinary rates of solidification.the colloidalgroupsare allowed sufficient time to'coalesce into aggregates ofmicroscopic dimensions with the result that normal structures ensue, butwith ultrarapid rates of solidification the tendency is for thecolloidal state to persist after solidi- .fication, in which case theresultant structures cease to be normal and approximate to modified.

Such an ultra-rapid rate of cooling may be produced, say by quenchingfrom the molten state in iced brine. In actual practice it is notusually possible to chill in this manner nor, generally speaking, in anyother manner with a degree of rapidity suificiently great to obtainmodified structures, especially in the case of sand castings, butrecourse must be had to some suitable moditying agent and in our viewthe true function of such a modifying agentis that of a colloidprotector and the process of modification consists simply in the more orless complete stabilization ,of the colloidal state.

Whether this be the true explanation or not is immaterial but it assistsin understanding and in predicting.

Thus on our theory it best results will only be obtained in practice ifthe correct quantity of.modifying agent,

be employed and experience has proved that this is the case.

. This can be illustrated by adding progressively increased quantitiesof a modifying agent to a previously normal a lloy, whence it is seenthat the structure s improved up to a certain percentage addition butafter that deterioration or reversion sets in.

'We have also found by a series of hardness determinations according tothe Brinell method that accompanying the structural change there is amarked improvement in the mechanical properties and theseexperimentsalso emphasize the desirab lity of employing the colloidprotector 1n the correct amount.

What happens in the case of one same; of

scribed.

have difierently from aluminum-silicon alloys although they'might beexpected to be substantially similar. Sodium chloride will normalize amodifiedaluminumsilicon alloy. Sodium hydroxide will modify a normalalloy but sodium hydroxide will normalize a modified aluminum- 1ronalloy and sodium chloride must be used for modifying the normal alloy.

" The lnventlon conslsts 1n a process for physically varying thestructure of aluminum-silicon alloys which comprises the ad- "dltlon-tothe molten alloy of a peroxide of alkali metal, for example sodiumperox- The invention f'also consists in the methods of modifying alloyshereinafter de- The invention further consists in im proved or modifiedalloys prepared ac cording to the process described herein.

In carrying the invention into effect in ,One form applied by way ofexample to an I aluminum-silicon alloy containing 87 per cent aluminumand 13 per cent silicon, we

"introduce a small percentage, say for complete conversion about 3 percent by weight,

Of sodium; peroxide, into the molten alloy preferably in small portionsat a time because the reaction is strongly exothermic. The sodiumperoxide is forced to the bottom of the melt which need not be raisedhigher in temperature than "is usual in aluminum foundry practical Themelt is then vigor- I ously stirred; a strong reaction takes place 40:and the melt rises in temperature with each addition. After skimming oilthe dross and slag and cooling to the temperature desired for pouringthe metal is poured in the a usual way, preferably in a chill mould. In

the case of alloys which are already partlally modified or partiallyreverted v smaller percentages of modifying agent may be employed andthis should prefer- In place of peroxide of sodium-peroxide ofpotasslumor other alkali metal peroxide or mixtures of such peroxides may beused.

The proportion of modifying agent may be varied, an experiment readilyshow-.

ing the best quantity to employ. Our experiments have shown that in thecase of an aluminum-silicon alloy containing a medi um amount ofsilicon, namely, 11 per cent, the Brinell hardness number increased byabout 8 to 9 per cent as the quantity of modifier -was increased fromnothing to about 3 to 5per cent and then decreased so that when theaddition was as high as 7 per cent there was a marked reduction in theBrinellnumber, other tests as well as the Brinell test according to theusual methods of testing alloys but the Brinell. number gives-a fairindication. i

Modification or structural change brings with it a marked improvement.in the mechanical properties.

The inve' tion is not limited to treatment of the above alloys as longas the alloysare largely predominant inalumin um and contain silicon asthe other principal ingredi-. ent Whether there are also appreciableamounts of other elements of aluminum and silicon or not.

It is better 'to make Further, the proportions given above are notlimiting proportions.

The alloys to be modified should not 'contain more than a smallpercentage of iron and should not be held long in the molten conditionafter the addition so that, for example, the operation of casting shouldbe carried out either in chill moulds or in sand moulds provided withsuitable chills so as to accelerate the rate of solidification of themetal in the heavier portions of the castother principal ingredientwhich consists in the addition to the molten alloy of sodium peroxide. vv

'3.-A method for physically varying the structure of alloys largelypredominant in aluminum arid contaming silicon as. the other principalingredient whichconsists in the introduction intothe molten alloy atatemperature only slightly above the melting point of the alloy of aperoxide of an alkali metal.

4. A method for physically varying the structure of alloys largelypredominant H1 aluminum and containing silicon as the other principalingredient which consists in the introduction into-the molten alloy at atemperature only slightly above the melting point of the alloy of sodiumperoxide.

In testimony whereof we have signed our names to this specification.

